Automotive vehicles with an internal combustion engine have an exhaust system that includes a pathway for exhaust gas to move away from the engine. The temperature of the exhaust gases ranges from ambient temperature, when the engine has not been run recently, to higher than 1000.degree. Celsius. Frequently used in these exhaust systems is an Exhaust Gas Oxygen (EGO) sensor assembly, which allows for a determination of a rich or lean air/fuel ratio.
The sensing element of an EGO sensor consists of a dense oxygen-conducting zirconia (ZrO.sub.2) ceramic, most commonly in the shape of a thimble, with porous platinum electrodes, one on the outside and the other on the inside surfaces of the thimble. The outside electrode is covered with a porous protective layer, e.g., from spinel. This sensing element is mounted onto a spark-plug type of a structure that seals the inside of the thimble from the outside of the thimble. When the EGO sensor is mounted onto the exhaust manifold of an engine, the outer electrode is exposed to the exhaust stream whereas the inner electrode is exposed to the ambient air as a reference oxygen atmosphere. When the air/fuel ratio is lean, the EGO sensor voltage output has a small value (e.g. 50 mV) because the oxygen partial pressure in the exhaust gas is not too different from the oxygen pressure in the air. When the air/fuel is rich, the EGO voltage output is large (e.g., 700-900 mV) because the thermodynamic equilibrium oxygen partial pressure of the exhaust gas is many orders of magnitude smaller than that of the air reference. Consequently, when the air/fuel ratio is changed through the stoichiometric value, the EGO sensor output changes abruptly between a large and a small value. This sensor output signal is obtained by means of an associated set of electrical output leads. This signal is then used by the engine control system to adjust the air-to-fuel mixture being supplied to the combustion chambers of the engine to the desired value, generally very close to the stoichiometric air/fuel ratio.
Most current EGO sensors also include a heater that is inserted in the air reference. The heater assists the zirconia sensor, a heated exhaust gas oxygen (HEGO) sensor, in making more precise measurements over a wide range of exhaust gas temperatures, especially when the exhaust gas temperature is low. The addition of the heater also helps to decrease the light-off time of the sensor, that is the time that it takes for the sensor to reach the minimum temperature for proper operation.
While engine systems utilizing catalysts and HEGO sensors with stoichiometric air/fuel control generally work very well, the hydrocarbon emissions during the cold start phase of engine operation account for approximately half of the total hydrocarbon emissions for new ultra-low emissions vehicles. Consequently, several methods have been developed for reducing cold start hydrocarbon emissions. Many of these are more effective if feedback control of the air/fuel ratio can be accomplished immediately after start-up of the engine. This requires HEGO sensors with reduced light-off times as compared to today's HEGO sensors.
Reduction of light-off times of thimble-type HEGO sensors has been accomplished through the use of high power heaters. However, these times are still generally longer than fifteen seconds because of the large size of the zirconia thimble and the poor thermal coupling of the heater to the thimble. Even with the more recent planar-types of zirconia HEGO sensor configurations, which have smaller thermal mass and better thermal coupling between the heater and zirconia, the light-off times are generally longer than ten seconds.
One method for further decreasing light-off times while using only small or modest heating power is to substantially decrease the size of the zirconia sensing element. This can be more easily accomplished by eliminating the air reference. An example of a sensor without an air reference is illustrated in U.S. Pat. No. 4,304,652 to Chiba et al. This reference describes a planar type sensor have a zirconia layer with one catalytic electrode covered with a porous gas-diffusing layer and one non-catalytic electrode. A DC current is applied to this device which then produces an output voltage that is indicative of air/fuel ratios in the lean range or in the rich range depending on the direction of the current. Good operation of this sensor, however, depends critically on the stability of the two electrodes, especially of the non-catalytic electrode. Unfortunately, this is difficult to accomplish.
It is thus desirable to have an exhaust gas oxygen sensor without an air reference, which not only produces a signal when the air/fuel ratio is changed through stoichiometry, but also that that signal is reproducible and stable over long periods of time, and has minimal thermal mass to reduce the power consumption of the sensor assembly.